Production and treatment of pearlitic malleable cast iron



United States Patent 2,995,441 PRODUCTION AND TREATMENT OF PEARLITICMALLEABLE CAST IRON Werner Riilrel, Velbert, Rhineland, Germany,assignor to Walter Overholf, Velbert, Rhineland, Germany, a corporationof Germany No Drawing. Filed Apr. 28, 1958, Ser. No. 732,768 Claimspriority, application Germany Apr. 29, 1957 10 Claims. (Cl. 75-123)2.80-3.l% carbon 0.40-1.20% silicon 0.30-1.20% manganese, preferably0.30-0.60% manga nese 0.l2-0.35% sulphur Up to 0.10% phosphorous0.01-0.5% titanium 0.00-0.5% Chromium Balance iron C-haracten'm'ngfeatures of this novel composition are the titanium content thereof andalso the relative proportion of the manganese, sulphur, titanium andchromium.

Cast iron of the aforementioned composition is adapted for conversion tomalleable iron having good structural state and excellent strengthproperties in the annealed and also in the hardened or ennobled hardenedand drawn state. Any of the heating treatments heretofore employed inthe production of malleable iron can be employed in malleabilizing thecast iron of the invention. A feature of the malleable iron of theinvention is that it can have an extremely fine lamellar pearliticstructure or an extremely fine globular pearlitic structure.

The malleable iron can in general be formed from the cast iron ingreatly reduced annealing times. This factor can be of greatsignificance since the required annealing time for malleabilizing iscommonly an important or even controlling factor in respect to theeconomics of production of the material. A further advantage of thenovel malleable iron is that it can be fusion welded. Hence, themalleable iron is suited for use in fusion welded structures.

The cast iron of the invention suitable for conversion to malleableiron, is produced by charging a cupola having a non-acid lining ofbasicity from neutral to semibasic, with ferrous material and slaggingmaterial suitable to form a semi-basic slag. The charge within thecupola is maintained at a temperature suflieient to fuse the ferrousmaterial and form a slag of the slagging material, and the slag ismaintained semi-basic. Ferrous melt formed within the cupola iswithdrawn and cast to form a cast metal of the aforementionedcomposition which is suitable for malleabilizing according to theinvention. The composition of the ferrous melt, and therefore the castiron, is controlled by controlling the composition of the cupola liningand/or the compositions of the materials charged to the cupola.

The titanium content of the cast iron can be obtained by the use oftitaniferrous pig iron or other titanium bearing material.

Preferably, the cupola employed is a semi-basic or neutral cold-blastcupola, and corresponding semi-basic slag regulation is employed.

By the expression semi-basic or neutral cold-blast cupola, is meantacupola furance which, deviating from the normal acid lining, has alining which is semi-basic or neutral, i.e., inthe case of thesemi-basic lining, there is an addition of A1 0 or AMO -containing ores,namely 8 to 25% and preferably 18 to 25% A1 0 based on the liningmaterial. in small quantities, MgO and TiO In the case of the neutrallining, graphite in more or less coarse pieces is included in theOtherwise, it consists essentially of silica. In this case also the SiO,generally contains small amounts of TiO,.

By semi-basic slagging is meant the preparation of a slag, the finalanalysis of which is approximately the following:

8-30%, preferably 10-20% A1 0, 40-60%, preferably 45-55% SiO; 20-35%,preferably 20-30% CaO As remaining components, the following will as arule be present:

About- 1-5% MnO 1-4% FeO Up to 3% TiO, Up to 2% M30 1-2.5% P 0 Very finepearlitic malleable iron can be produced from the cast iron by annealingtreatments heretofore known for the production of malleable iron fromcast iron but which treatments are in the case of the inventionconsiderably shortened but are of a time suflicient to form the desiredpearlitic structure. While being malleabilized, the ledeburite structureof the thoroughly white cast iron (having no graphite in this state)disintegrates into fine lamellar pearlite and temper carbon. Whenannealing is continued, the lamellar pearlite is by and by convertedinto globular pearlite.

This is achieved without any prolonged annealing near the transformationtemperature and without recourse to the process of temperaturealternation above and below the transformation temperature, but may beefiected at the malleabilizing temperature in short time.

By the expression fine lamellar pearlitic structure is meant a structurehaving cementite and ferrite in form of very thin and short lamellas; bythe expression fine globular pearlitic structure is meant a structure inwhich the lamellas of the cementite are spheroidized.

The presence in the metal composition of the invention of the alloyelements, particularly the titanium, chromium and sulphur, has theeifect that it favors the formation of ledeburite, but at the same timethe ledeburite formed from the metal compositions decomposes extremelyeasily upon the annealing so that the advantage is obtained thatannealing times are relatively short.

The metallic composition of the invention is particularly well suited totreatment designed to provide in the castings, surfaces free of ferriteand which are completely pearlitic. To obtain surfaces of this nature,according to the invention, the castings are treated while present in aneutral environment and preferably in a salt bath. This manner oftreatment is to be considered in contradistinction to the customarymethods of malleabilizing in an oxidizing atmosphere in which a temperore is used as a source of oxygen and this oxygen burns the carbonpresent at the skin zone of the casting, and also in contradistinctionto the nowcommonly used treatment of gas malleabilizing.

The salt bath should if possible have an evaporation point above 950 C.Suitable salt baths are those containing sodium carbonate, chlorides,particularly barium Patented Aug. 8, 1961. t

The balance consists of SiO; and,

3 chloride and possibly sodium cyanide and potassium cyanide. There hasbeen found to be suitable a bath additionally containing molybdenumsulfide, particularly molybdenum disulfide, as well as of sodium sulfiteand barium sulfite, singly or in any combination. In this way malleablecast iron having a ferrite-free skin zone can be obtained.

The treatment temperature in the salt bath is between 760 and 1100 C.and preferably between about 950 and 1000 C. One can proceed in variousmanners. It is possible to effect the entire treatment in the salt bathor in a plurality of salt baths, the first of which for instance is setat a temperature between 950 and 1000 C. and another bath at 760 to 950'C. It is furthermore possible to provide only a short time of stay inthe salt bath, for instance 30 to 80 minutes, and to thereupon place thetreated pieces in a furnace. v This furnace should have a controllableatmosphere, exclusion of air or inert gas. The stay in the salt baths orin the furnace is'dependent on the cross-section of the pieces to betreated. However, in the case of exclusive salt-bath treatment, theduration will be less than 10 hours, and depending on the height of thetemperature, even considerably less. Also upon after-treatment in afurnace, of for instance 760 to 850 C., the periods of time areconsiderably reduced and are at most between about 8 and 15, hours.

The tensile strength of the malleable cast iron treated in this manneris, without heat treatment, between about 50 and 65 kg./mm. with a yieldpoint of about 32 to 48 kg./mm. and an elongation of about 3 to 20%(L=3d). By heat treatment, yield points of between 70 and .120 kg./mm.'at tensile strengths of 80 to 160 kgJmmP'can be obtained.

The cast iron can also be excellently carburized and/or surface-hardenedor through-hardened. In particular when decarburizing of the surface isto be avoided, it is advisable to effect not only themalleabilizing butalso the heating to hardening temperature in the salt bath. One canproceed in the manner that hardening is effected at a suitabletemperature directly from the bath in which the malleabilizing wasefiected.

Referring again to the composition of the cast iron and malleable ironof the invention, it will be noted that the stoichiometricsulphur-manganese ratio common to malleable castings is substantiallyhigher than is true for the metallic material of the invention, and thatthe material of the invention is of relatively high sulphur content andat the same time, of low manganese content. While in the case of.previously known malleable cast iron compositions, the rule ofapproximation applies that the manganese content should be at leasttwice or at least three times the sulphur content. In the case of thematerial of the invention, lower ratios are employed. Thus according tothe invention, the manganese content is preferably less than about 1.7times the sulphur content.

In the case of compositions of the invention of low sulphur contents,i.e., between about 0.12 and 0.2%, a chromium content of 0.02 to 0.5%and preferablyseveral tenths of a percent is provided, and where thesulphur content is higher than 0.2, the chromium content is advisedlykept very low, for example from to 0.09%.

The relationship of titanium, manganese and sulphur in the compositioncan be expressed by the following formulae:

Ti (percent max.)=0.2% Mn Mn (percent max.)=l.7% S

The phosphorus content of the composition of the invention is preferablymaintained below the upper limit of 0.1% and it is highly desirable ifthis content is within the limits of 0.03 to about 0.07.

The production of cast iron and the production of malleable ironaccording to the invention will now be described with reference toworking examples. These examples are ofiered in order to illustrate inthe inven- 4 tion and are not intended to set forth the limits of theinvention.

Example I I The operation is carried out in a cupola, the lining ofwhich, in addition to Si0,, also contains about 15% A1 0; 1% TiO; 4% C2% M30 1.5% FeO 1.5% MnO The mix also contains the recycle material ofthe titaniferous grade produced here. Basic-Bessemer steel scrap wasalso added. In this case, one refrained from using FeMn or FeSi, whichmay definitely enter into consideration in other cases. The mixaccordingly contained the following percentages by weight:

Percent Pig ir n 14 Recycle material 42 Scrap 44 In addition to this,there is included as slag-forming addition per 350 kg. of metalliccharge 20 kg. CaCO; 8 kg. A1303, and 15 kg. $10,

In this connection, there was obtained a malleable crude cast iron ofthe following composition:

2.9% carbon 0.63% silicon 0.31% manganese 0.24% sulphur 0.06% phosphorus0.08% titanium 0.05% chromium The final slag analysis belonging to itwas as follows:

56% sio, 9.8% Ago, 23.64% 0110 4.31% no 0.84% "no, 0.7% MnO 1.35% MgO1.3% 150,

From this crude cast iron there were produced machine-part castings, thepieces being made with a wall thickness of 5 to 45 mm., and these partswere malleabilized at temperatures of 960 C. in a slightly oxidizingatmosphere for 60 hours, plus heating-up time and cooling-down time.Comparable parts of normal malleable cast iron required an annealingtime of about hours. The parts to be hardened, after the malleabilizingprocess, were heated to 850 C., held at this temperature for 3 to 5minutes and then quenched in cold water. In order to increase thetoughness, the parts were tempered to 380 0., time of tempering 3 to 5minutes. The technological properties prior to the heating and quenchingwere as follows:

Yield point kg./mm.- 32-36 Tensile strength kg./mm..... 52-58 Elongationpercent.. 6-8

with a cross section of 12 mm.

The technological properties after the tempering were as follows:

Yield point kg./mm.' 80-90 Strength "kg/mm?" 120-135 Elongation pcrcent1-3 Another part of this crude casting was malleabilized in a salt bath,namely in a bath having the following composition:

The malleabilizing temperature was 950' C. and its length 1 hour,followed by annealing for 12 hours at 850' C. At this time the yieldpoint was 48-52 lag/mm, the tensile strength 66-75 kg./mm. and theelongation 4 to 5%.

The matrix in this case was of very finely globularpearlitic structurewith extremely finely divided temper carbon. No special heat treatmentwas efiected here since the strength properties were suflicient for thepurpose of use even without special heat treatment. Otherwise, it couldhave been followed directly by a heat treatment which would have givencomparably higher values for the yield point and tensile strength withthe same treatment than in the case of the above described cast ironmalleabilized in the furnace.

Example II In this example, the melt was formed in the presence of aneutral lining.

In a cupola of approximately identical dimensions to that used inExample I, there was provided in the hearth and nozzle zones a lining ofabout 60% graphite Balance substantially SiO,

A mix was selected with titaniferous pigiron as in Example I.

To this there were added 52% recycle material and 38% chromiferous steelscrap For each 350 kg. of this mix there were added kg. CaCO; 6 kg. A10, 15 kg. SiO,

6 The malleabilizing was effected by annealing at about 960 C. as in thefirst example. The technological properties of this cast iron were, inmalleabil-ized state:

Yield point", kg./mm. 36-38 Tensile strength "kg/mm-.. 51-56 Elongationpercent.. 7-11 with a test bar of 12 mm. o, referred to a measurementlength of the test bar of 3 xdiameter.

I claim: a

1. Cast iron consisting essentially of the following constituents in theamounts indicated based on the total of said constituents:

2.8-3.1% carbon 0.4-1.2% silcon 0.3-1.2% manganese 0.l2-0.35% sulphur Upto 0.10% phosphorus 0.01-0.5% titanium 0.00-0.5% chromium Balance ironfurther characterized by the fact that the manganese content is lessthan 1.7 times the sulfur content, and the titanium content is less than0.2 times the manganese content.

2. Cast iron according to claim 1, with the proviso that for a sulphurcontent less than about 0.2%, the chromium content is at least 0.2% andfor a sulphur content greater than about 0.2% the chromium content isless than about 0.09%.

3. Cast iron according to claim 1, the manganese content being less thanabout 1.7 times the sulphur content, the titanium content being lessthan about 0.2 times the manganese content and the phosphorus contentbeing in the ranage of about 0.03 to about 0.07%.

4. The method of producing cast iron suitable for conversion topearlitic malleable iron by annealing for a relatively short period oftime which comprises charging to a cupola having a non-acid lining ofbasicity from neutral to semi-basic, ferrous metal, and inorganicslagging material suitable to form a slag, maintaining the cupola at atemperature sufiicient to fuse the ferrous material and form a slag ofthe slagging material, and collecting a ferrous melt in the cupola, thecompositions of the ferrous material, cupola lining and slaggingmaterial bei: g such that at the temperature maintained in the cupolathe melt consists essentially of the following constituents in theamounts indicated based on the total of said constituents:

2.8-3.1% carbon 0.4-1.2% silicon 0.3-1.2% manganese 0.124135% sulphur Upto 0.10% phosphorus 0.01-0.5% titanium 0.00-0.5% chromium Balance ironannealing said cast iron for a relatively short annealing time, wherebyfine pearlitic malleable iron is formed.

5. The method of claim 4, wherein the slag comprises 8 to 30% alumina,40 to silica, and 20 to 35% calcium oxide.

6. The method of claim 4, wherein the said sl' neutral and comprisesgraphite silica and titaniur oxide.

7. The method of claim 4 wherein the slag additi ly contains 1-5% MnO,l-4% FeO, up to 3% Ti( to 2% MgO and 1-2.5% phosphorus pentoxide.

8. The method of producing a very fine globular pearlitic malleable ironby employing a relatively short holding time at the hardeningtemperature which comprises heating at the hardening temperature apreviously malleabilized iron essentially of the following constituentsin the amounts indicated based on the total of said constituents:

2.8-3.196 carbon (IA-1.2% silicon 03-12% manganese 0.12-0.35% sulphur Upto 0.10% phosphorus 0.01-0.5% titanium 0.00-0.5% chromium Balance ironfor a time sufficient to form a very fine globular pearlitic I indicatedbased on the total of said constituents:

2.8-3.1% carbon 0.4-1.2% silicon (LB-1.2% manganese 0.l2-0.35% sulphurUp to 0.10% phosphorus 0.01-0.5% titanium 0.00-0.5% chromium Balanceiron in a salt bath for about 30 to 80 minutes at a malleabilizingtemperature and thereafter heating in a furnace in the presence of aneutral environment atv a malle abilizing temperature.

10. The methodot producing pearlitic malleable iron suitable for use infusion welded structures which comprises heating a cast iron consistingessentially of the following constituents in the amounts indicatedbasedon the total of said constituents:

2.84.196 carbon 0.4-1.296 silicon 03-12% manganese (LIZ-0.35% sulphur Upto 0.10% phosphorus (ml-0.5% titanium 0.000.5% chromium Balance ironFletcher et a1 June 15, 1926 Schwartz Aug. 30, 1927 FOREIGN PATENTSFrance Mar. 22, 1956

1. CAST IRON CONSISTING ESSENTIALLY OF THE FOLLOWING CONSTITUTENTS INTHE AMOUNTS INDICATED BASED ON THE TOTAL OF SAID CONSTITUTES: 2.8-3.1%CARBON 0.4-1.2% SILCON 0.3-1.2% MANGANESE 0.12-0.35% SULPHUR UP TO 0.10%PHOSPHORUS 0.01-0.5% TITANIUM 0.00-0.5% CHROMIUM BALANCE IRON